1. Field of the Invention
The present invention relates to a motorcycle and more particularly to an arrangement of the air/gas mixture intake device and exhaust pipe. In addition, the present invention relates to the arrangement of an exhaust pipe for a motorcycle having a heat exchanger, such as a radiator, mounted on the front of the motorcycle.
2. Description of Background Art
In a motorcycle, an exhaust pipe extends from an engine rearwardly along a vehicle body. Generally, in the case of a two stroke cycle engine, in order to enhance a scavenging efficiency by utilizing an inertia effect of an exhaust gas within the exhaust pipe, the exhaust pipe increases its volume gradually from its inlet of the exhaust gas and after reaching a maximum diameter, decreases its volume gradually so as to become a considerably slender form at the outlet thereof.
FIGS. 1 and 2 are a side view and a cross-sectional view, respectively, of a conventional off the road motorcycle. A vehicle body frame F comprises a head pipe 01 at the front portion thereof, a main pipe 02 extending rearwardly of the upper portion of the head pipe 01, a down tube 03 extending obliquely downwardly and rearwardly of the lower portion of the head pipe 01, a pair of right and left under pipes 04 which are connected to the down tube 03 and extend rearwardly at the lower portion of the vehicle body, and a pair of right and left center pipes 05 adapted to connect each end of the under pipes 04 with the rear end of the main pipe 02.
A front wheel 07 is supported on the vehicle body frame F through the intermediary of a front fork 06 which is rotatably mounted relative to the head pipe 01. A rear wheel 09 is carried on the rear end of a rear fork 08 which is pivoted at the rearward lower portion of the vehicle body frame F. A rear cushion 011 is provided at the center of the width of the vehicle body. A seat 012 is mounted above the cushion. A fuel tank 013 is mounted in front of the seat 012 straddling the main pipe 02. A two stroke cycle engine 014 is mounted in front of the rear cushion 011 and under the fuel tank 013. A exhaust pipe 015 of the engine 014 extends forwardly of the engine cylinder, curves downwardly laterally, ascends upwardly along the left lateral side of the vehicle body, and then extends further rearwardly along the left lower edge of the seat 012. The forward portion of the exhaust pipe 015 is an expansion chamber 015a of a larger diameter. The rearward portion thereof is tapered to form a smaller diameter portion 015b and is connected with a silencer 016. Longitudinally extending on the right side of the engine 014 and the rear cushion 011 is an air/gas mixture intake device which includes a suction pipe 017, a carburetor 018, a connecting pipe 019 and an air cleaner 020. In addition, a kick pedal 091 is provided for starting the engine.
FIG. 5 illustrates another cross-sectional view of a conventional motorcycle wherein the fuel tank 043 includes a right portion 04b and a left portion 04a. The left portion 04a is smaller in size relative to the right portion 04b to permit the exhaust pipe 071 to be disposed adjacent to the left portion 04a. A kick pedal 092 is provided for starting the engine.
In Japanese provisional patent Publication No. 61-163074, a motorcycle is disclosed in which an exhaust pipe is positioned along a main frame at the same level as a fuel tank. As illustrated in FIG. 6, in this motorcycle, a fuel tank 041 is provided only on one side of a main frame 051. An exhaust pipe 072 is provided on the other side displaced from the fuel tank 041. In another example, as illustrated in FIG. 7, the fuel tank 042 is divided into a pair of right and left tanks 0111, 0112. Each tank 0111, 0112 is mounted on the vehicle body from the respective lateral sides. An exhaust pipe 073 is disposed adjacent to the central portion of the gas tanks. A main frame 052 is positioned within the gas tanks 0111, 0112.
In the above-referred motorcycle, as the right and left center pipes 05, 05 are gathered at a higher level and are connected to the rear end of the main pipe 02, the width (span) between the center pipes 05, 05 becomes narrower gradually in proportion as the center pipes 05, 05 project upwardly. Therefore, through the intake device (the connecting tube 019) located at a comparatively low position is able to be arranged inside the center pipes 05, the exhaust pipe 015 located at a high position must be arranged outside the center pipe 05. In order to prevent the exhaust pipe 015 from projecting from the lateral side of the vehicle body, the center pipe 05 at the left side is formed in the upper portion thereof to deviate more inwardly than the upper portion of the right side center pipe 05. Accordingly, an unbalanced arrangement will be developed in the vehicle body frame. Thus, to improve the unbalanced arrangement in strength and rigidity, it is necessary to increase the weight of the vehicle body frame.
Further, even if the right and left center pipe 05, 05 are formed unsymmetrical, the exhaust pipe 015 will project laterally outwardly. Thus, the riding comfort of the driver will suffer therefrom until the width of the seat 012 is expanded or the side cover 021 which covers the exhaust pipe 015 is expanded to project laterally outwardly. Generally, with respect to a riding posture of the driver, such a riding posture wherein the legs are freely stretched from the narrower seat position to the wider step position is better.
With respect to FIG. 5, the bottom of the left portion 04a of the fuel tank must be positioned relatively high. Thus, a volume of the fuel tank will be reduced and the weight of the fuel tank will be unbalanced. This result also occurs with respect to FIG. 6. In FIG. 7, the unbalanced in weight of the fuel tank may be avoided. However, there is an increase in component elements and time for manufacturing because two fuel tanks are required to be mounted on the motorcycle.
With respect to a third embodiment of the present invention, a motorcycle includes a water-cooled engine having a radiator mounted at the front side of the vehicle body. Air passes over the radiator to thereby cool the radiator. In addition, motorcycles are known which include an oil cooler or the like as a heat exchanger which is provided in the front portion of the vehicle body.
On the other hand, in a two stroke cycle engine, the inside of an exhaust pipe is maintained at a negative pressure during a scavenging operation. This is caused by the inertia effect of the exhaust gas therein in order to expel the exhaust gas and thus to improve scavenging efficiency. In another aspect, by utilizing a reflection wave, a positive pressure is created at the position of an exhaust port just before the exhaust port is closed to press back fresh air which is about to blow through. Therefore, one form of exhaust pipe increases its volume gradually from an inlet of the exhaust gas, becomes a maximum volume and thereafter decreases its volume gradually so as to be considerably tapered at an outlet thereof. Further, for obtaining a necessary length for the above operation, the exhaust pipe is curved in a large curvature from the inlet of the exhaust gas communicating with the exhaust port of the engine, extends downwardly, then laterally of the vehicle body and turns upwardly thereby extending along the bottom of the fuel tank above the engine or extending rearwardly substantially at the same level as the fuel tank.